Organic EL display devices have organic EL elements and are arranged as pixels in a matrix and display by individually controlling the emission of the organic EL elements of the respective pixels. Organic EL display devices include an active type and a passive type. The active type organic EL display device, has associated with each pixel, a pixel or drive circuit for controlling current through the corresponding organic EL element. Active matrix types of drives are better for performing high definition display.
FIG. 1 shows an example of the pixel circuit of the active type organic EL display device. A drive TFT 1 is a p-channel type having its source connected to a power supply PVdd and its drain connected to an anode of an organic EL element 2. A cathode of the organic EL element 2 is connected to a cathode power supply CV.
A gate of the drive TFT 1 is connected to a source of an n-channel type selection TFT 3. A drain of the selection transistor 3 is connected to a data line Data which extends in a vertical direction, and a gate thereof is connected to a gate line Gate which extends in a horizontal direction. One end of a retention capacitor C, the other end of which is connected to a capacitor power supply Vsc, is connected to the gate of the drive TFT 1. Such pixels are arranged in a matrix in a display area of the organic EL panel.
TFT 3 is turned on when the gate line Gate is set to a high level. At this time, when an image signal representing luminance of the pixel is applied to the data line Data, a voltage of the image signal is held in the retention capacitor C and applied to the gate of the drive TFT 1. A gate voltage of the drive TFT 1 is controlled by the image signal, and such gate voltage controls the current flowing to the organic EL elements 2. The gate voltage of the drive TFT 1 is held at a level by virtue of the retention capacitor C even after the selection TFT 3 is turned off.
An amount of emitted light of the organic EL element 2 is substantially proportional to its drive current. Therefore, the organic EL element 2 emits light according to the image signal.
An adjustment of the luminance of the organic EL panel is proposed by, for example, Japanese Patent Laid-Open Publication No. 2002-215094 (hereinafter referred to as the patent publication 1). This patent publication 1 shows that when luminance data has a prescribed level or more, the amount of current to the organic EL elements is reduced. But, it does not suggest any idea of adjusting an offset voltage.
The drive TFT 1 is turned on when the gate voltage becomes lower than the voltage of the power supply PVdd by a threshold voltage Vth or more (Vgs>Vth). Then, an offset voltage corresponding to the voltage Vth is added to an image signal to be supplied to the gate of the drive TFT 1 so that a drain current starts to flow in the vicinity of a black level of the image. The amplitude of the applied image signal is such as to provide a prescribed luminance in the vicinity of a white level. Thus, the organic EL element 2 emits light with a luminance according to the image signal.
However, the Vth of the drive TFT 1 is variable among the respective panels and also varies depending on temperature and lowers with an increase in temperature. When the Vth lowers, black in the displayed image becomes whitish to decrease contrast. Also, the luminance as a whole is increased, and current consumption increases. As a result of the increase in current consumption, there are problem such as the organic EL elements degrading quickly.